By John Davison and John Irish BEIRUT/GENEVA (Reuters) - A senior official from Syria's main opposition group said on Monday that a fragile international attempt to halt nearly five years of fighting was in danger of collapse because of attacks by government forces. The cessation of hostilities drawn up by Washington and Moscow faced "complete nullification" because Syrian government attacks were violating the agreement, the official of the Saudi-backed opposition High Negotiations Committee (HNC) said. The Kremlin said the process was under way, although it had always been clear it would not be easy.